1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display which is capable of strengthening adhesion between the upper and lower plates of a panel with a high aperture ratio to which and organic protective film is applied, and a fabrication method thereof.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls the light transmissivity in accordance with video signals by liquid crystal cells arranged in a matrix pattern to thereby display a picture corresponding to the video signals on a liquid crystal panel. To this end, the LCD includes a liquid crystal panel having liquid crystal cells arranged in an active matrix type, and driving integrated circuits (ICs) for driving the liquid crystal cells. The ICs are usually manufactured in the chip type, and mounted on a tape carrier package in the case of a tape automated bonding (TAB) system, or mounted on the surface of the liquid crystal panel in the case of a chips on glass (COG) system. In the case of a TAB system, the driving ICs are electrically connected, via the TCP, to a pad provided on the liquid crystal panel.
FIG. 1 shows a plan view of a liquid crystal panel 2 having the structure of oppositely adhering a lower plate 4 to an upper plate 6. Referring to FIG. 1, the liquid crystal panel 2 includes a picture display part 8 having liquid crystal cells arranged in a matrix pattern, gate pads 12 and data pads 14 connected to gate lines and data lines of the picture display part 8, respectively. In the picture display part 8, the data lines supplied with video signals are intersected with gate lines supplied with a scanning signal, that is, a gate signal at the lower plate 4. At the intersections, thin film transistors for switching the liquid crystal cells and pixel electrodes connected to the thin film transistors to drive the liquid crystal cells are provided. Color filters coated separately for each area by a black matrix and a common transparent electrode, which is a counterpart of the pixel electrode, are provided at the upper plate 6. The lower plate 4 and the upper plate 6 having the configuration as described above are spaced apart by a spacer and include a cell gap inside thereof. The cell gap is filled with a liquid crystal material. The lower plate 4 is adhered to the upper plate 6 by a sealant coated on the seal 10 positioned at the outside of the picture display part 8. The gate pad 12 and the data pad 14 are located at the edge of the lower plate 4 which is not overlapped with the upper plate 6. The gate pad 12 applies gate signals applied from the gate driving ICs to the gate lines of the picture display part 8. The data pad 14 applies video signals applied from the data driving ICs to the data lines of the picture display part 8.
A protective film for protecting the metal electrodes and the thin film transistors is entirely coated on the lower plate 4. The pixel electrodes are formed on the protective film for each cell area. An inorganic material such as SiNx or SiOx has been conventionally used as the protective film. Since the inorganic protective film has a high dielectric constant and is formed by a vapor deposition technique, it has a drawback in that it is difficult to increase the height thereof. Accordingly, the pixel electrodes and the data lines having the inorganic film therebetween must keep a constant horizontal interval of, for example, 3 to 5 μm so as to minimize a coupling effect caused by a parasitic capacitor. As a result, the size of pixel electrodes having an influence on the aperture ratio of the liquid crystal cell is decreased to have a low aperture ratio. In order to solve this problem, an organic material with a relatively low dielectric constant has been used as the protective film recently. Since this organic protective film has a low dielectric constant of about 2.7 and is formed by a spin coating method, it has an advantage in that it can be formed to a desired height or thickness. Such an organic protective film minimizes a capacitance value of the parasitic capacitor, so that the pixel electrode can be overlapped with the data lines without any horizontal interval therebetween. As a result, the size of pixel electrodes is enlarged to improve the aperture ratio.
When the lower and upper plates of the liquid crystal display with such a high aperture ratio are adhered with a sealant, the sealant usually contacts the organic protective film of the lower plate. However, the organic protective film has a weak adhesive characteristic with respect to a sealant such as epoxy resin. Also, the organic protective film has a weak adhesive characteristic with respect to a gate insulating film positioned at the lower part thereof. Due to this, when the strength of the organic protective film itself is weak, or an adhesion between the organic protective film and the sealant or the gate insulating film is poor, a crack or a layer separation phenomenon is caused by a minute impact at the poor adhesive part. As a result, there is a problem in that liquid crystal is leaked through a poor adhesive part between the sealant and the gate insulating film of the lower and upper portions of the organic protective film. Hereinafter, the problem in the conventional liquid crystal display will be described in detail with reference to the accompanying drawings.
FIG. 2 is an enlarged view of a part of a data link crossed by the seal in FIG. 1. In FIG. 2, the data link 16 is a connecting part between the data pad 14 and the data line of the picture display part and is formed along with the data pad 14 and the data line. A semiconductor pattern 18 is extended into the data pad 14 at the lower portion of the data link 16. The seal 10 coated with a sealant is located in a direction crossing the data link 16. The data pad 14 contacts a transparent film 17 formed on the organic protective film through a contact hole defined in the organic protective film. The transparent film 17 is responsible for protecting the data pad, formed as a metal electrode, and for preventing oxidation of the metal electrode during the repetition of a TCP adhesion required for the TAB process.
FIG. 3A shows a section of the seal 10 in FIG. 2 taken along line A-A′, and FIG. 3B shows a section of the seal 10 taken along line B-B′. In FIGS. 3A and 3B, the lower plate 4 has such a structure that a gate insulating film 22, a semiconductor pattern 18 and a data link 16 are sequentially disposed on a lower glass 20 and an organic protective film 24 is entirely coated thereon. In FIG. 3B, a liquid crystal 32 of the picture display part has been injected into the right side of a sealant 11. The upper plate 6 has a structure such that a color filter and a black matrix 26 are formed on an upper glass 30 and a common transparent electrode is entirely coated thereon. The lower plate 4 is adhered to the upper plate 6 with the sealant 11. In this case, the sealant 11 is adhered to the organic protective film 24, thereby providing weak adhesion. Also, the organic protective film 24 has a weak adhesion to the gate insulating film 22 at the lower portion thereof. There is a problem in that, when the adhesion between the organic protective film 24 and the sealant 11 or the gate insulating film 22 is weak, a crack is caused by a minute impact to leak a liquid crystal.
FIG. 4 is an enlarged view of a part of a gate link crossed by the seal in FIG. 1. In FIG. 4, the gate link 34 is a connecting part between the gate pad 12 and the gate line of the picture display part and are formed along with the gate pad 12 and the gate line. The gate pad 12 contacts a transparent film 17 formed on the organic protective film through a contact hole 19 formed by way of the gate insulating film and the organic protective film. The transparent film 17 is responsible for protecting a metal electrode as the gate pad. The seal 10 coated with a sealant is located in a direction crossing the gate link 34.
FIG. 5A shows a section of the seal 10 in FIG. 4 taken along line A-A′, and FIG. 5B shows a section of the seal 10 taken along line B-B′. In FIGS. 5A and 5B, the lower plate 4 has a structure such that the gate link 34 and a gate insulating film 22 are sequentially disposed on a lower glass 20 and an organic protective film 24 is entirely coated thereon. The upper plate 6 has such a structure that a color filter and a black matrix 26 are formed on an upper glass 30 and a common transparent electrode is entirely coated thereon. The lower plate 4 is adhered to the upper plate 6 with the sealant 11. In this case, the sealant 11 is adhered to the organic protective film 24, thereby providing a weak adhesion. Also, the organic protective film 24 has a weak adhesion to the gate insulating film 22 at the lower portion thereof. There is a problem in that, when adhesion between the organic protective film 24 and the sealant 11 or the gate insulating film 22 is weak, a crack is caused by a minute impact to leak liquid crystal material.
As a result, since a liquid crystal display with a high aperture ratio to which the conventional organic protective film is applied has a weak adhesion characteristic between the organic protective film and the sealant or the gate insulating film, there is a problem in that a crack is easily caused by a slight exterior impact and hence liquid crystal is leaked through the crack.